1. Field of the Invention
The present invention relates most generally to a process for the electroless plating of easily reducible metals onto ultrafine particles, a process for making alloy mixtures using the ultrafine particles having plating thereon and to the unique products produced thereby such as, metal powders of ultrafine colloidal sized particles with cores or centers with a dense and continuous plating of at least one metal and metal articles of manufacture having a plurality of ultrafine particles dispersed substantially evenly through the metal article. Such electroless plating is achieved through careful and accurate control of such parameters as the feed rates of the various solutions, the control of pH of the solution, the temperature, pressure and the rate of agitation of the solution in which the plating is taking place.
2. Description of the Prior Art
Applicant has searched both chemical and metallurgical abstracts, for specific systems such as gold-silica and silver-tin oxide, and has also searched under gold in addition to reviewing material by specific authors. No references to the application of electroless plating of noble metals on colloids or colloidal sized particles was found. No references to 14 K gold alloys which has been "extended" using an oxide metal or carbon filler was found. Also there was no process found for the electroless plating of ultrafine particles in an aqueous slurry and at temperatures below 90.degree. C. and providing for the simultaneous and separate adding of both the complexed metal ion and an appropriate reducing agent. No prior art has been developed which discloses the process or the products disclosed herein by Applicant.
Some Patents which are representative of the general field of art in which the invention may be classified are discussed.
The German Patent 1,143,372 to Mackiw et,al discloses that the powdered material to be metallized by treatment with a metal salt solution in reducing medium is treated in an ammoniacal metal salt solution of Os, Rh, Ru, Ir, Au, Pt, Pd, Ag, Cu, As, Pb, Sn, Ni or Co with reducing gases at a partial pressure of less greater than 4 atm. and a temperature of greater than 90.degree. C.
Christini et al (U.S. Pat. No. 3,940,512) concerns electroless plating in a "tumble barrel". The particles being plated or coated are extremely large relative to the particles being plated according to the present invention. The particles referred to in Christini et al as being sized from 0.05 to 100 microns are particles which are in the plating solution but are not the "articles" being plated upon. There is no discussion of nor is there a showing of concern for aggregation of plated articles. There is no need for such concern because of the very large relative size of the articles being plated as compared to the particles being plated in the present invention.
U.S. Pat. No. 4,403,001 concerns silver electroless plating on diamond. Suffice it to say that there is a great difference in the behavior of amorphous, spherical oxide particles in colloidal solution and a suspension of diamonds. In aqueous solution, colloidal particles of oxides can and will coalesce whereas, in aqueous suspensions, crystalline diamond will not. For this and other reasons it is easy to coat or plate diamonds with silver without gross aggregation and the formation of so-called "popcorn balls". Such is not the case for colloidal aquasols of silica or tin oxide which are some of the particles plated by the processes of the present invention.
U.S. Pat. No. 3,218,192 concerns coating phosphorus with nickel or cobalt. The particles are 1 to 400 microns are dispersed in a suspension. It is important to note that suspensions and colloidal solutions are very different and require very different treatment. A thorough and detailed discussion of colloidal chemistry can be found in R. K. Iler, "The Chemistry of Silica, Solubility, Polymerization Colloid and Surface Properties, and Biochemistry", Wiley Interscience 1979. In the prior art there is no teaching of or appreciation for the critical nature of stability of particles. The treatment of diamond and other crystalline particles and graphite and red phosphorus is not the same as the treatment of, for example colloidal silica or complex oxides.
U.S. Pat. Nos. 4,353,741, 4,240,830, 4,403,506 all disclose methods which involve the electroless plating of small particles in a solution. For example, U.S. Pat. No. 4,353,741 discloses a process in which a slurry of particles is coated with silver. In the process, a reducing agent such as hydrazine is added to a slurry of particles in a solution containing a silver salt. It should be noted that the solutions are not added simultaneously.
U.S. Pat. No. 3,556,839 discloses a process in which diamond particles are coated with nickel or cobalt in an electroless process. In the process, a metal salt solution and a reducing agent are utilized.
U.S. Pat. No. 3,062,680 discloses a process for the electroless coating of fine particles. The particles are dispersed in a metal solution and the metal salt is reduced by means of a gaseous reducing agent.
U.S. Pat. No. 2,853,398 discloses a method of making metal plated particles. In the process, particles of one metal are dispersed in a solution containing a dissolved metal salt and the metal is precipitated onto the particles by reduction with a gas. An additional reducing agent, such as hydrazine, is used to treat the particles to assure depostion of the metal onto the particles. It is said that the particles may vary in size from 1-200 microns or smaller. The reaction temperatures in the process are high, plating concentrations are high and the use of 0.33 molar nickel solution (38.4 gram of nickel in 2,000 milliliter--see Example 1) will cause the negatively charged silica sol particles or the negatively charged tin oxide particles to aggregate prior to the coating process. The process taught would destroy a colloid before it could be electrolessly plated with metal. Of similar interest are U.S. Pat. Nos. 2,853,401 and 2,853,403.
U.S. Pat. No. 2,424,085 discloses a process of making catalyst particles by applying a silver coating onto the particles. The silver is applied by reducing a silver salt in a solution containing a reducing agent such as hydrazine.
The prior art does not disclose methods or processes for the uniform dispersion of ultrafine colloidal particles (less than about 20 microns) such as silica throughout a metal or an alloy. It would be of considerable value to be able to disperse such ultrafine particles throughout a noble metal such as gold thereby "extending" the gold. That is to say, being able to obtain more use from or make more articles from a given amount of gold. It is important that the extended gold or other metal have substantially the same appearance and working characteristics as the unextended gold or other metal. It would also be desirable to be able to make a metalliferous powder of ultrafine particles which powder particles would have, for example, a core or center of silica or a base metal having plated thereon, a dense and continuous coating of a metal. This metallic powder would then be useful in the making of metal articles which have the core material evenly dispersed throughout the article, by cold or hot pressing or by casting with alloys or recastable alloy mixtures made with the powder particles. Again, the prior art does not disclose such products or processes for making such products. Prior art attempts to plate small particles have been limited to particles considerably larger than those which can be plated by the processes of the present invention. It has been attempted in the past to extend gold by the incorporation of refractory materials, by powder metallurgy blending techniques, but such materials cannot be cast.
Electrical contactors and/or connectors are presently made in the United States using silver with cadmium oxide added to suppress the arc which form during use. Cadmium is toxic, and its use in contacts has been banned in some countries. The Japanese are using tin oxide, and are investigating other substitutes, including the oxides of tin, indium, nickel, manganese, aluminum and iron. There is also an active research program in Germany to find a substitute for cadmium.
The silver-tin oxide materials are prepared either by internal oxidation or by powder metallurgy techniques. In the former process and depending on the concentrations used, an oxide case may form around the silver-tin alloy. This prevents further oxidation of the tin, and limits the concentration of oxide that can be included in the composite. Contacts prepared by powder metallurgy are less brittle than the internally oxidized counterparts but the tin oxides in powder metallurgy materials grow as needles, which degrades the properties (ductility) of the contact material.
There is need in the United States to eliminate cadmium oxide from electrical contacts. The process and products herein disclosed suggests an entirely new and novel approach to filling this need. By the process of this invention, silver-tin oxide contacts can be made by electroless plating onto ultrafine tin oxide particles a coating of silver resulting in a powder which can then be formed into silver-tin oxide electrical contacts. Such powders are much more uniform in microstructure. Such uniformity means that the metals produced therefrom are more ductile and the properties of the composites are more uniform within the metal structure. Higher concentration of tin oxide in silver can be prepared. There appears to be no upper limit for the relative amount of tin oxide. Volume loadings as high as 50 percent have been made. There is no evidence that needle like tin oxide particles are formed on the thermal aging of the powders. Under proper conditions the powders can be processed by casting. Casting has many advantages in the forming of articles for electrical use.